This application claims priority under Title 35 of United States Code, xc2xa7119 to Federal Republic of Germany Application No. 198 55 478.8 filed Dec. 1, 1998.
The present invention relates to a method and an apparatus for the optical detection of a contrast line. The invention further relates to a numerically controlled machine, in particular a machine tool, a robot or a coordinate measuring machine, as well as to a hand-held device for the optical detection of a contrast line.
In known apparatuses for the optical detection of contrast lines, the measurement surface is scanned by a laser beam which is deflected, for example, by a swivellably or rotatably disposed, electrically controlled mirror. Depending on the mirror position, the laser beam reflected from the mirror is directed to the measurement surface in different directions. In such an apparatus, one disadvantage is that the angle of incidence of the laser beam hitting the measurement surface is not identical for all measurement points, but changes according to the disposition of the mirror. In this way, it becomes necessary to take the angle of incidence into account in the assessment.
An apparatus is known from DE-OS 40 29 339 A1 where the point light beam incident to the measurement surface is deflected in relation to the incident direction originally given by the beam source without the angle of incidence being changed thereby. In this known prior apparatus, the point light beam is subjected to a parallel shift within a deflection apparatus, but not to a change in the angle of incidence. Using such an apparatus, it is possible to inspect a plane measurement surface exactly, with the apparatus being moved over the measurement surface at an approximately constant distance.
It is the object of the present invention to improve an apparatus and a method for the optical detection of a contrast line.
This object is solved by a method for the optical detection of a contrast line in which the area or spatial coordinates of the contrast line are detected, with the detection being performed by means of a scanner, preferably a laser scanner, which scans the contrast line in a linear manner. In such a coordinate measuring system, both the surfaces of a measurement object and characteristic lines on the measurement object can be detected in a non-contact manner in their area or spatial extension. The contrast lines can be, for example, construction lines or scribed lines, with both narrow lines, whose width is below the scan width of the apparatus in accordance with the invention, and wide lines or contrast differences being detectable. The contrast difference can also be formed by a change in the brightness of the contrast surface or also by color differences on the measurement surface. It is equally feasible for the contrast lines to be formed by contours or shape differences.
It is particularly advantageous if the detection of the contrast line is effected by means of a distance signal and/or by means of an intensity signal. During the digitizing of contrast lines, both the distance and the intensity signals are assessed. The distance signal serves to regulate the distance during the movement procedure and to compute the coordinates of the measurement point. Contrast transitions are detected using the intensity signal. The requirement for this is that the corresponding apparatus detector, which is preferably designed as an infrared detector, can distinguish the contrast difference. Changes in the intensity signal are produced, for example, by the transitions at surfaces of different colors and by light/dark or dark/light transitions, tapes stuck on materials such as plasticine and ureol or marks with an Edding color pen on paper.
In a further aspect of the present invention, it is provided that the scan line of the scanner or the laser scanner is aligned perpendicularly to the contrast line.
Here, the scan line can be tracked in such a way that it is always perpendicular to the contrast line even when the latter is curved, with the tracking preferably being effected by the rotation of the measurement head of the scanner or laser scanner.
In another aspect of the present invention, it is provided that the detection of the contrast line is performed by the intensity of the beam emitted by the scanner or laser scanner being controlled in such a way that the intensity of the reflected beam on the detector of the scanner or laser scanner always assumes a constant value, with the intensity of the emitted beam serving as a measurement value for the detection of the contrast line.
The intensity signal is taken, for example, from the intensity control circuit of the laser electronics, which intensity control circuit ensures that the laser, advantageously designed as a triangulation laser, provides a constant distance signal in wide areas despite the changing material finish of the surface to be inspected and despite the changing reflection angle of the laser beam in relation to the surface. High intensities suggest poor reflectance, low intensities good reflectance.
It is particularly advantageous if the number of linear scans (scan lines) increases as the radius of curvature of the contrast line decreases. In this way, it becomes possible to provide a number of measurement points with small radii of curvature in order to detect the profile correspondingly precisely. Thus, the measurement apparatus is reduced correspondingly in speed on tight radii of curvature, which allows the detection of more measurement points in these regions.
The present invention further relates to an apparatus for the optical detection of a contrast line having a radiation source, preferably a light source for coherent radiation, preferably a laser diode, a deflection apparatus for the deflection of the beam emitted from the radiation source and a detector for the detection of the reflected beams, with the deflection apparatus being designed in such a way that the beam emitted from the radiation source or the laser diode can be displaced, preferably displaced in parallel and with the area or spatial coordinates of the contrast line being detectable by means of the beams detected by the detector.
It is particularly advantageous if the radiation source or the laser diode, the deflection apparatus and the detector are disposed in a measurement head. The apparatus or the measurement head can be designed rotatably around the axis of the beam emitted from the radiation source or the laser diode. The measurement head can be designed in such a way that it can be disposed on a machine tool or coordinate measuring machine or any other robot. If the line to be examined or detected possesses a curved course, the apparatus or the measurement head can be guided using the axis of rotation in such a way that its longitudinal axis is always aligned in parallel to the tangent on the line at the point measured. In this state, the scan line is aligned perpendicularly to the contrast line to be examined.
It is particularly advantageous if the detector is designed as a line sensor. It is also feasible for an area sensor to be used. The sensor in accordance with the invention is advantageously designed as a position sensitive detector (PSD) or a charge coupled device (CCD).
In accordance with a preferred embodiment of the present invention, a control unit is provided by means of which the intensity of the beam emitted from the radiation source or the laser diode can be modified in such a way that the intensity of the beam detected by the detector always assumes a constant value.
In a further aspect of the present invention, it is provided that the deflection apparatus is defined in such a way that the scan width is 10 to 12 mm.
It is particularly advantageous if the deflection apparatus is designed as a rotating prism. In this way, it is achieved that depending on the rotation position of the prism, a deflection of the beams is effected without this changing the incident direction. By means of such a rotating prism it is possible to perform a linear scanning of the contrast line. Instead of a prism, it is equally feasible to use any other deflection apparatuses by means of which a linear scan is possible. A swivellable mirror can also be used.
The present invention further relates to a numerically controlled machine, particularly a machine tool, a robot or a coordinate measuring machine, which possesses an apparatus for the optical detection of a contrast line. Such NC machines usually possess three linear and two rotational axes (horizontal arm or portal). By means of the preferably provided rotational facility of the measurement head or the apparatus in accordance with the invention around the axis of the beam emitted from the radiation source, another rotational axis is created which allows the apparatus to be aligned always in the desired manner over the contrast line.
The present invention further relates to a hand-held device for the optical detection of a contrast line which possesses an apparatus. Here, it is advantageous to provide a detection device by means of which the position of the hand-held device can be detected exactly. Here, the hand-held device can possess a plurality of light sources whose positions can be detected using corresponding detection devices. With such a hand-held device; the position of the contrast line relative to the hand-held device can be detected by means of the apparatus in accordance with the invention and the position of the hand-held device relative to a position detection device can be detected by means of a corresponding position detection device. By the combination of both measurement results, the area or spatial structure of the contrast line can be determined.